Ground-detector and cut-out.



No. 825,286. PATENTED JULY 10, 1906. E. F. W. ALEXANDERSON.

GROUND DETECTOR AND OUT-OUT.

APPLICATION FILED SEPT. 14.1904.

l V/ mess esr //7 z/enzfar m [fr/7st F M! Aexanderson UNITED STATESPATENT OFFICE.

ERNST F. W. ALEXANDERSON,

OF SCHENECTADY, NEWYORK, ASSIGNOR TO GENERAL ELECTRIC COMPANY, ACORPORATION OF NEW YORK. GROUND-DETECTOR AND CUT-OUT.

Specification of Letters Patent.

Patented July 10, 1906.

To all whom it may concern.-

Be it known that I, ERNST F. W. ALEXAN- DERSON, a subject of the King ofSweden and Norway, residing at Schenectady, county of Schenectady, Stateof New York, have invented certain new and useful Improvements inGround-Detectors and Cut-Outs, of which the following is aspecification.

My invention relates to protection against grounds of three-phasesystems having a grounded neutral point; and its object is to provideprotective means for detecting grounds or cutting out grounded lines orboth, which is more sensitive and reliable than the devices heretoforeemployed.

It has been customary heretofore to protect against grounds by means ofoverload devices or static devices. The former of these from their verynature will not respond unless the ground or leakage current is ofsufficient amount to overload the lines of the generator or other devicewhich is to be protected, while static devices are for the most partunreliable.

By my invention-I provide a protective device for three-phase systemswith grounded neutral point, which while possessing the reliability ofoverload devices will nevertheless respond to aground or leakage currentof an amount which maybe as small a fraction of the overload-current asdesired. To accomplish this end, I take advantage of the fact that thealgebraic sum of the currents at any instant in the three branches of athree-phase system is always equal to zero regardless of whether theload is balanced or not. A device to which is supplied the algebraic sumof the currents in the phases will consequently remain inoperative, nomatter how unevenly divided or how great the load may be. In athree-phase system having a grounded neutral point, however, if aconductor of one of the phases becomes grounded a ground or leakagecurrent will flow, which will traverse the conductor of the groundedphase alone, the circuit being completed by the earth instead of by theconductors of the other phases. The balance of the currents in thephases is thus destroyed. A device to which is supplied the algebraicsum of the currents in the three phases will now be energized.

Evidently since the device will respond only when a ground exists it maybe made as sensitive as desired, so as to operate ona leakagecurrentequal to a very small fraction only of the full-load current.

My invention will best be understood by reference to the accompanyingdrawings, in

which- Figure 1 shows diagrammatically a protective arrangement for athree-phase generator with a grounded neutral point arranged inaccordance with my invention, and Fig. 2 shows myinvention applied tothe protection of arallel feeders in a three-phase system wit groundedneutral point.

Referring first to Fig. 1, A represents a generator or other source ofthree-phase alternat ing current with the neutral point grounded, thecircuit of which it is desired to open upon the occurrence of a groundupon any one of the phases. S represents a switch adapted to open thecircuit of ithe generator A, and 8 represents a tripping-coil for theswitch. The series transformers T T T are included in the three phases,and the secondaries of the transformers are all connected in paralleland to the terminals of a relay R. As long as no ground exists upon aconductor of any of the phases the current in the secondary of one ofthe transformers will be equal and opposite to the algebraic sum of thecurrents in the secondaries of the other two transformers, andconsequentlyno current will flow through the actuating-coil of relay R.This is trueregardless of whether the load on the three phases isbalanced or not. Now if one of the conductors of the three phasesbecomes grounded a circuit will exist comprising the conductor of thatphase alone and the earth return, so that the algebraic sum of thecurrents in the three phases will no longer be zero. The resultantcurrent will consequently flow through the winding of the relay R, whichwill therefore be energized, so as to draw up its core, closing thecircuit of battery I) through the tripping-coil s and allowing switch Sto open, so as to break the cir-' cuit of the generator A.

Referring now to Fig. 2, I have shown my invention applied to theprotection of para lel feeders in a three-phase system with line L tothe fault.

grounded neutral point. In this figure, A and A represent two generatorshaving their neutral points grounded and connected to the stationbus-bars B. B represents thebus-ba'rs at the receiving end orsubstation. The generatingstation is connected to the substation throughthe three parallel lines L, L and L These parallel lines or feeders areprovided at the generating ends the relays R, R and R respectively, andat the receiving ends with similar relays R to R each relay beingenergized from a group of series transformers arranged as heretoforedescribed. The relay R is provided with a dash-pot d, and the otherrelays are similarly provided, the object being to cause the relays tooperate on the well-known inverseti'me-element principle, so that thegreater the current in the relay-coil the quicker the relay will move.Now assume a ground upon one of the conductors of any one of the threeparallel lines. For instance, suppose a ground occurs upon one of theconductors of the line L Current will be supplied to the ound directlyfrom the bus-bars B throu h the .line L and indirectly through the linesand L and back from the receiving end through line L to the ground. Thebalance of current in the secon aries of each group of seriestransformers will be destroyed, and each relay-core will start to move.The relative speed with which the cores move will depend somewhat uponthe position of the round on the line L Thus, for instance, if t eground in the line L occurs very near the generating-station nearly allthe leakage-current will flow directly from the generator bus-bars Bthrough line L to the ground, and the amount of current flowing to theground indirectl' through the lines L and L will be muc smaller, owingto the greater resistance of the circuit. The relay-coil R will underthese circumstances be most strongly energized and will be the first toclose, thereby disconnecting line L from the bus-bars B. The switchescontrolled by the relays and connecting the feeders to the bus-bars areshown diagrammatically at S S &c. After the line L is disconnected fromthe generator bus-barsB a leakage-current will still flow through linesL and L andback through The leakage or fault current in line L will betwice as great as in lines L or L so that rela -coil R will be morestrongly energized t an any of the other four, and consequently will bethe first to close, disconnecting the line L from the receiving bus-barsB. The faulty line L is thus disconnected at both ends, and the servicecontinues without interruption. Oh the other hand, if the fault occurson the line If near the receiving bus-bars B, so that the resistancefrom the generator bus-bars B to the fault is practically the same byway of each of the three lines, the relay R will be the most stronglyenergized of all of the relays and wi l consequently close first,disconnecting the line L from the receiving bus-bars B. The only pathfor the fault-current is now directly from the generating bus-barsthrough line L to the ground. Relay R is consequent}? the only one thatis energized and closes, isconnecting the line L from the eneratingbusbars B. Thus on line L the aulty line alone will be cut out withoutregard to the position of the fault on the line and without interruptionof the service. As explained above, the relays may be arranged torespond to a fault-current very much ess than the full-load current, andconsequently more adequate protection against grounds is secured thanwould be possible by the ordinary overload devices.

It is evident that my invention has other applications besides thoseshown, and consequently particular construction and arrangement of partsshown in the drawings; but I aim in the appended claims to cover allmodifications which are within the scope of my invention.

What I claim as new, and desire to secure by Letters Patent of theUnited States, is

1. In combination, a three-phase circuit I do not desire to limit myselfto the having the neutral point grounded, an electroresponsive device,and means for supplying to said device a current proportiona to thealgebraic sum of the currents in the three phases.

2. In combination, a three-phase circuit having the neutral pointgrounded, an electroresponsi've device, means for supplying to saiddevice a current proportional to the algebraic sum of the currents inthe three phases, and a switch controlled by said device adapted to opensaid-circuit.

3. In combination, a threehase circuit having the neutral point grouned, a switch adapted to 0 en said circuit, a tripping-coil for saidswitch, a relay controlling the circuit of said tripping-coil, and meansfor supplying to said relay a current proportional to the algebraic sumof the currents in the three phases of said circuit.

4. In a three-phase system with grounded neutral point, parallel lines,electroresponsive devices energized respectively by currentsproportional to the algebraic sum of the currents in the three phases ofthe several lines, and switches controlled respectively by theseveraldevices and each adapted to open the circuit of one of said lines at oneend thereof.

5. In a three-phase system with grounded neutral point, parallel lines,relays at each IIO end of each line, connections for supplyin to Inwitness whereof I have hereunto set my each relay a current proportionalto the a gehand this 10th day of September, 1904. braic sum of thecurrents in the three hases at that end of the line, means control ed byERNST ALEXANDERSON' each relay for opening the circuit of the lineWitnesses: at that end, and means for retarding the BENJAMIN B. HULL,

movement of the relays. HELEN ORFORD.

